This invention relates to semiconductor memory devices.
More particularly, the present invention relates to semiconductor random access memory devices that utilize a magnetic field.
Non-volatile memory devices are an extremely important component in electronic systems. FLASH is the major non-volatile memory device in use today. Typical non-volatile memory devices use charges trapped in a floating oxide layer to store information. Disadvantages of FLASH memory include high voltage requirements and slow program and erase times. Also, FLASH memory has a poor write endurance of 104-106 cycles before memory failure. In addition, to maintain reasonable data retention, the scaling of the gate oxide is restricted by the tunneling barrier seen by the electrons. Hence, FLASH memory is limited in the dimensions to which it can be scaled.
To overcome these shortcomings, magnetic memory devices are being evaluated. One such device is magnetoresistive RAM (hereinafter referred to as xe2x80x9cMRAMxe2x80x9d). To be commercially practical, however, MRAM must have comparable memory density to current memory technologies, be scalable for future generations, operate at low voltages, have low power consumption, and have competitive read/write speeds.
For an MRAM device, the stability of the nonvolatile memory state, the repeatability of the read/write cycles, and the memory element-to-element switching field uniformity are three of the most important aspects of its design characteristics. A memory state in MRAM is not maintained by power, but rather by the direction of the magnetic moment vector. Storing data is accomplished by applying magnetic fields and causing a magnetic material in a MRAM device to be magnetized into either of two possible memory states. Recalling data is accomplished by sensing the resistive differences in the MRAM device between the two states. The magnetic fields for writing are created by passing currents through strip lines external to the magnetic structure or through the magnetic structures themselves.
As the lateral dimension of an MRAM device decreases, three problems occur. First, the switching field increases for a given shape and film thickness, requiring a larger magnetic field to switch. Second, the total switching volume is reduced so that the energy barrier for reversal decreases. The energy barrier refers to the amount of energy needed to switch the magnetic moment vector from one state to the other. The energy barrier determines the data retention and error rate of the MRAM device and unintended reversals can occur due to thermofluctuations (superparamagnetism) if the barrier is too small. A major problem with having a small energy barrier is that it becomes extremely difficult to selectively switch one MRAM device in an array. Selectablility allows switching without inadvertently switching other MRAM devices. Finally, because the switching field is produced by shape, the switching field becomes more sensitive to shape variations as the MRAM device decreases in size. With photolithography scaling becoming more difficult at smaller dimensions, MRAM devices will have difficulty maintaining tight switching distributions.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved method of writing to a magnetoresistive random access memory device.
It is an object of the present invention to provide a new and improved method of writing to a magnetoresistive random access memory device which is highly selectable.
It is another object of the present invention to provide a new and improved method of writing to a magnetoresistive random access memory device which has an improved error rate.
It is another object of the present invention to provide a new and improved method of writing to a magnetoresistive random access memory device which has a switching field that is less dependant on shape.
To achieve the objects and advantages specified above and others, a method of writing to a scalable magnetoresistive memory array is disclosed. The memory array includes a number of scalable magnetoresistive memory devices. For simplicity, we will look at how the writing method applies to a single MRAM device, but it will be understood that the writing method applies to any number of MRAM devices.
The MRAM device used to illustrate the writing method includes a word line and a digit line positioned adjacent to a magnetoresistive memory element. The magnetoresistive memory element includes a pinned magnetic region positioned adjacent to the digit line. A tunneling barrier is positioned on the pinned magnetic region. A free magnetic region is then positioned on the tunneling barrier and adjacent to the word line. In the preferred embodiment, the pinned magnetic region has a resultant magnetic moment vector that is fixed in a preferred direction. Also, in the preferred embodiment, the free magnetic region includes synthetic anti-ferromagnetic (hereinafter referred to as xe2x80x9cSAFxe2x80x9d) layer material. The synthetic anti-ferromagnetic layer material includes N anti-ferromagnetically coupled layers of a ferromagnetic material, where N is a whole number greater than or equal to two. The N layers define a magnetic switching volume that can be adjusted by changing N. In the preferred embodiment, the N ferromagnetic layers are anti-ferromagnetically coupled by sandwiching an anti-ferromagnetic coupling spacer layer between each adjacent ferromagnetic layer. Further, each N layer has a moment adjusted to provide an optimized writing mode.
In the preferred embodiment, N is equal to two so that the synthetic anti-ferromagnetic layer material is a tri-layer structure of a ferromagnetic layer/anti-ferromagnetic coupling spacer layer/ferromagnetic layer. The two ferromagnetic layers in the tri-layer structure have magnetic moment vectors M1 and M2, respectively, and the magnetic moment vectors are usually oriented anti-parallel by the coupling of the anti-ferromagnetic coupling spacer layer. Anti-ferromagnetic coupling is also generated by the magnetostatic fields of the layers in the MRAM structure. Therefore, the spacer layer need not necessarily provide any additional antiferromagnetic coupling beyond eliminating the ferromagnetic coupling between the two magnetic layers. More information as to the MRAM device used to illustrate the writing method can be found in a copending U.S. Patent Application entitled xe2x80x9cMagnetoresistance Random Access Memory for Improved Scalabilityxe2x80x9d filed of even date herewith, and incorporated herein by reference.
The magnetic moment vectors in the two ferromagnetic layers in the MRAM device can have different thicknesses or material to provide a resultant magnetic moment vector given by xcex94M=(M2xe2x88x92M1) and a sub-layer moment fractional balance ratio,       M    br    =                    (                              M            2                    -                      M            1                          )                    (                              M            2                    +                      M            1                          )              =                            Δ          ⁢                      xe2x80x83                    ⁢          M                          M          total                    .      
The resultant magnetic moment vector of the tri-layer structure is free to rotate with an applied magnetic field. In zero field the resultant magnetic moment vector will be stable in a direction, determined by the magnetic anisotropy, that is either parallel or anti-parallel with respect to the resultant magnetic moment vector of the pinned reference layer. It will be understood that the term xe2x80x9cresultant magnetic moment vectorxe2x80x9d is used only for purposes of this description and for the case of totally balanced moments, the resultant magnetic moment vector can be zero in the absence of a magnetic field. As described below, only the sub-layer magnetic moment vectors adjacent to the tunnel barrier determine the state of the memory.
The current through the MRAM device depends on the tunneling magnetoresistance, which is governed by the relative orientation of the magnetic moment vectors of the free and pinned layers directly adjacent to the tunneling barrier. If the magnetic moment vectors are parallel, then the MRAM device resistance is low and a voltage bias will induce a larger current through the device. This state is defined as a xe2x80x9c1xe2x80x9d. If the magnetic moment vectors are anti-parallel, then the MRAM device resistance is high and an applied voltage bias will induce a smaller current through the device. This state is defined as a xe2x80x9c0xe2x80x9d. It will be understood that these definitions are arbitrary and could be reversed, but are used in this example for illustrative purposes. Thus, in magnetoresistive memory, data storage is accomplished by applying magnetic fields that cause the magnetic moment vectors in the MRAM device to be orientated either one of parallel and anti-parallel directions relative to the magnetic moment vector in the pinned reference layer.
The method of writing to the scalable MRAM device relies on the phenomenon of xe2x80x9cspin-flopxe2x80x9d for a nearly balanced SAF tri-layer structure. Here, the term xe2x80x9cnearly balancedxe2x80x9d is defined such that the magnitude of the sub-layer moment fractional balance ratio is in the range 0xe2x89xa6|Mbr|xe2x89xa60.1. The spin-flop phenomenon lowers the total magnetic energy in an applied field by rotating the magnetic moment vectors of the ferromagnetic layers so that they are nominally orthogonal to the applied field direction but still predominantly anti-parallel to one another. The rotation, or flop, combined with a small deflection of each ferromagnetic magnetic moment vector in the direction of the applied field accounts for the decrease in total magnetic energy.
In general, using the flop phenomenon and a timed pulse sequence, the MRAM device can be written to using two distinct modes; a direct write mode or a toggle write mode. These modes are achieved using the same timed pulse sequence as will be described, but differ in the choice of magnetic sub-layer moment and polarity and magnitude of the magnetic field applied.
Each writing method has its advantages. For example, when using the direct write mode, there is no need to determine the initial state of the MRAM device because the state is only switched if the state being written is different from the state that is stored. Although the direct writing method does not require knowledge of the state of the MRAM device before the writing sequence is initiated, it does require changing the polarity of both the word and digit line depending on which state is desired.
When using the toggle writing method, there is a need to determine the initial state of the MRAM device before writing because the state will be switched every time the same polarity pulse sequence is generated from both the word and digit lines. Thus, the toggle write mode works by reading the stored memory state and comparing that state with the new state to be written. After comparison, the MRAM device is only written to if the stored state and the new state are different.
The MRAM device is constructed such that the magnetic anisotropy axis is ideally at a 45xc2x0 angle to the word and digit lines. Hence, the magnetic moment vectors M1and M2 are oriented in a preferred direction at a 45xc2x0 angle to the directions of the word line and digit line at a time t0. As an example of the writing method, to switch the state of the MRAM device using either a direct or toggle write, the following current pulse sequence is used. At a time t1, the word current is increased and M1 and M2 begin to rotate either clockwise or counterclockwise, depending on the direction of the word current, to align themselves nominally orthogonal to the field direction due to the spin-flop effect. At a time t2, the digit current is switched on. The digit current flows in a direction such that M1 and M2 are further rotated in the same direction as the rotation caused by the digit line magnetic field. At this point in time, both the word line current and the digit line current are on, with M1and M2 being nominally orthogonal to the net magnetic field direction, which is 45xc2x0 with respect to the current lines.
It is important to realize that when only one current is on, the magnetic field will cause M1 and M2 to align nominally in a direction parallel to either the word line or digit line. However, if both currents are on, then M1 and M2 will align nominally orthogonal to a 45xc2x0 angle to the word line and digit line.
At a time t3, the word line current is switched off, so that M1 and M2 are being rotated only by the digit line magnetic field. At this point, M1 and M2 have generally been rotated past their hard-axis instability points. At a time t4, the digit line current is switched off and M1 and M2 will align along the preferred anisotropy axis. At this point in time, M1 and M2 have been rotated 180xc2x0 and the MRAM device has been switched. Thus, by sequentially switching the word and digit currents on and off, M1 and M2 of the MRAM device can be rotated by 180xc2x0 so that the state of the device is switched.